PCPH was initially identified as a proto-oncogene in the PI's laboratory on the basis of its frequent detection as an activated oncogene (mt-PCPH) in chemically initiated, tumorigenic Syrian hamster embryo cells. It was later shown that PCPH is highly conserved in vertebrates, and that the mt-PCPH oneogene synergized with the human H-ras oncogene in the transformation of murine NIH3T3 fibroblasts. The central objective of this continuation application is to investigate the normal PCPH function and to establish the mechanism(s) by which PCPH alterations contribute to tumor development. Results obtained during the current funding period have shown that: i) Both PCPH and mt-PCPH have a predominant intracellular location; ii) the hydrophobic C-terminal tail present in the mt-PCPH oncoprotein is a key determinant of its transforming activity; iii) expression of mt-PCPH, but not of PCPH, activates the ERK1 kinase; iv) both PCPH and mt-PCPH have intrinsic ATP diphosphohydrolase (apyrase) activity, v) mt- PCPH expression provides cell survival functions by partially depleting endogenous ATP pools to levels below those optimal for stress-induced signaling and apoptosis, and vi) expression ofPCPH polypeptides is frequently deregulated in neoplastic animal and human cells. We hypothesize that PCPH is a component of the cellular ATP-sensing machinery and functions by interacting with signaling circuits in charge of balancing endogenous ATP pools and external supplies, Alterations of these interactions by either mutational activation of PCPH into an oncoprotein (mt-PCPH) or by deregulation of PCPH expression contribute to tumor development. This hypothesis will be tested by several approaches: 1) Analysis of mechanisms of transformation by mt-PCPH or deregulated expression of PCPH; 2) Study of the involvement of PCPH in regulating cellular ATP pools; 3) Analysis of PCPH/mTOR functional interactions in yeast cells; and 4) Study of the participation of PCPH in human cancer. These studies will provide a fundamental understanding of the function and regulation of wild type and oncogenic PCPH in normal and malignant cells, and may have important implications for the diagnosis and management of human tumors.